It follows a competitive process run by Ofgem to get the “best deal possible” for the 108,000 domestic customers and 21,000 business customers after the small supplier ceased trading on Wednesday.
ScottishPower will honour all outstanding credit balances of the customers, including money owed to both existing and former domestic and business customers.
They will be contacted by the Big Six supplier over the coming days and again in a few weeks when the switch has been completed and their accounts are fully set up.
At that point, customers wishing to leave ScottishPower can do so.
Philippa Pickford, Ofgem’s Interim Director for Future Retail Markets said: “We are pleased to secure a deal with ScottishPower, where Extra Energy’s domestic and business customers will be offered a competitive tariff for their energy. Their credit balances will be honoured and their energy supply will continue as normal.
“Out advice for customers of Extra Energy is to wait until ScottishPower contacts you. They will give you more information about the tariff you are on and about your credit balance if you have one. Once the transfer has been completed, you can shop around for a better deal if you wish to.”